In efficient absorption systems, most internal heat can be theoretically regenerated (or recuperated) using a generator-absorber type of heat exchange (GAX). However GAX systems present three major practical and economical problems that prevent full realization of the theoretically possible heat regeneration: (1) heat exchanger finite temperature approaches .DELTA.T's; (2) mixing losses in the absorption process; and, (3) rectification loses in the generation process.
Heat exchanger .DELTA.T's cause large performance loss. It is difficult to reduce this loss because of the negative exponential relationship between .DELTA.T and 1/UA (the reciprocal of the heat transfer coefficient U times the heat exchange surface area A). An increase in UA yields a rapidly diminishing return in performance, therefore the costly increase in area A must be supplemented by a more economical increase in U in order to get an acceptable performance at a reasonable cost. This invention provides a means that allows a better heat transfer coefficient U.
Mixing losses occur when feeding cold vapor into the absorption process. The losses are in the form of temperature drops resulting from mixing two streams that are not in temperature and/or in concentration equilibrium. The temperature drops reduce available heat exchanger .DELTA.T and, most importantly, shift the absorption heat to a lower temperature range where it is unavailable for recuperation. Mixing losses in absorption processes can be minimized by using counterflow vapor-liquid contact. This counterflow arrangement is difficult to realize in practical and economical systems because of its low heat transfer coefficients and its dependence on gravity, resulting in a bulky size. The present invention reduces the need for gravity.
Rectification losses occur when the vapor leaving the generation process is too hot and contains too much absorbent species. The vapor heat is eventually dumped to the atmosphere and the residual absorbent absorbs and retains much of the available high pressure vapor for a turbine, or much of the available refrigerant in an evaporator. Rectification losses in generation processes can be minimized by using counterflow vapor-liquid contact. Again this counterflow arrangement is difficult to realize in practical and economical systems because of its low heat transfer coefficients and its bulky size.
The difficulties are compounded in a GAX heat exchanger because both the absorption and the generation processes require counterflow vapor-liquid contact. Since only gravity can be used economically to effect such counterflow arrangement and since the processes in a GAX cycle should be themselves counterflow (if they are to be in direct heat transfer coupling), only one of them can be satisfied. Both processes can be made counterflow, with additional costs for an intermediate heat transfer loop that separates and put the flows in their appropriate directions. This approach is very expensive (because it doubles the size of the heat exchanger and requires more pumps) and inefficient because of additional heat exchanger .DELTA.T's that negate the benefits of counterflow.
Vapor-liquid concurrent absorption and generation processes are desirable because of their independence of gravity which allows more flexibility in system packaging to satisfy market preference and because of possible increases in fluid velocities that improve heat transfer coefficients, therefore reducing surface area and cost.